1. Field of the Invention
The present invention relates to a method for forming a photosensitive polyimide pattern formed on a metal film for wiring or the like. The present invention also relates to electronic devices such as semiconductor devices and thin film magnetic devices including thin film magnetic heads and thin film inductors, each including a photosensitive polyimide pattern as an insulating layer on a conductor.
2. Description of the Related Art
In electronic devices such as semiconductor devices and thin film magnetic devices including thin film magnetic heads and thin film inductors, the use of organic materials as an interlayer insulation film and a passivation film has been investigated. Among such organic materials, polyimide is excellent in heat resistance, mechanical properties and electric properties such as dielectric constant.
In these years, photosensitive type organic materials have been mainly used because they are advantageous concerning processing. In particular, photosensitive polyimide is advantageous as a highly heat resistant resin compatible with high-precision processing by use of lithography.
In a method for obtaining polyimide pattern, a polyimide pattern is obtained as follows: a polyamide acid having photosensitive groups (a photosensitive polyimide precursor) is irradiated with ultraviolet light to form cross-linking, development with a developer is carried out to remove the unexposed area, and thereafter imide rings are formed by heat curing to yield a polyimide pattern.
As such a photosensitive polyimide precursor, there may be cited an ester bond type photosensitive polyimide precursor and an ion bond type photosensitive polyimide precursor. Known examples of the ester bond type photosensitive polyimide precursor include a compound having a structure represented by the following formula (1), in the structure of formula (1) ethyl methacrylate groups as photosensitive groups being esterically bonded to the carboxyl groups in a polyamide acid molecule (see Japanese Patent Publication No. 55-30207 or Japanese Patent Laid-Open No. 60-228537):

On the other hand, known examples of the ion bond type photosensitive polyimide precursor in which photosensitive groups are ionically coupled to the carboxyl groups in a polyamide acid molecule or the salts thereof, and include a compound having a structure represented by the following formula (2) (see Japanese Patent Publication No. 59-52822):

The above described ester bond type photosensitive polyimide precursor can provide a satisfactory patterning even when the patterning is carried out on the surface of a metal such as Cu; however, the stress exerted by the thus obtained polyimide film is large, causing troubles such that the substrate on which the pattern formation has been made is warped and delamination between layers is generated when multi-layer structure is formed. On the other hand, the ion bond type photosensitive polyimide precursor results in a small stress exerted by the polyimide film obtained therefrom; however, this type precursor is known to form a complex with a metal as represented by the following formula (3) when patterning is made on the surface of a metal such as Cu. Accordingly, there occurs a problem that the complex thus formed is insoluble in a developer to prevent formation of a satisfactory pattern. Consequently, it is difficult to form a polyimide pattern, exerting small stress, directly onto the surface of a metal.

Among the hitherto known methods to avoid this problem are a method in which a substrate with metal wiring and the like is subjected to ozone treatment or the like for oxidation to form an oxide layer on the surface of the metal wiring, then an ion bond type photosensitive polyimide precursor is applied onto the oxide layer, and thereafter the substrate is subjected to exposure, development and heat treatment to yield a polyimide pattern (see Japanese Patent Laid-Open No. 5-198561), and a method in which an organosilicon compound layer is formed on a substrate with metal wiring and the like, an ion bond type photosensitive polyimide precursor is applied onto the thus formed layer, thereafter the thus processed substrate is subjected to exposure, development and heat treatment to yield a polyimide pattern, the thus processed substrate is further heated to 400° C. to convert the organosilicon compound layer into a silicon dioxide layer, and thereafter the silicon dioxide layer on the exposed area is removed (see Japanese Patent Laid-Open No. 5-197159). However, any one of these methods is complicated in processing in the sense that oxides and organic matters remaining on the exposed surface need be removed.
Japanese Patent Laid-Open No. 8-314147 proposes a method in which the diffusion of the Cu ions into the polyimide film is suppressed by adopting a way in which before application of a polyimide precursor, a very thin polyimide film, more specifically, a 100 nm or less thick prebaked polyamide film is formed, and a polyimide precursor is applied onto the prebaked film to form a pattern. In this case, for the prebaked film as a lower layer, it is described that either a photosensitive or a non-photosensitive (soluble) polyimide precursor is able to be used; in Example 1 therein an ion bond type photosensitive polyimide precursor diluted with a solvent was applied and prebaked, then the ion bond type photosensitive polyimide precursor not diluted with a solvent was applied and prebaked, and the thus processed substrate was subjected to exposure and development to yield a polyimide pattern. However, according to this method, the thin prebaked film formed as the lower layer forms copper complexes, and hence the problem involving a remaining film still persists unsolved although the remaining film is thin. In Examples 2 and 3, a non-photosensitive polyimide precursor has been used, and accordingly the copper complex formation has not been caused; but the upper-layer photosensitive polyimide pattern is removed onto a mask, so that there is an apprehension that the developer may penetrate into the inside of the pattern. Further, in such a thin film, the penetration of copper ion cannot completely be suppressed, and accordingly, there is a problem that the upper layer photosensitive polyimide precursor is involved in complex formation, and development failure is thereby caused.